Batteries are devices that convert chemical energy contained in active materials directly into electrical energy by means of an oxidization-reduction electrochemical reaction involving the transfer of electrons from one material to another. Lead-acid batteries are one of the most common kinds of batteries. Such a battery includes positive and negative lead electrodes and a mixture of sulfuric acid (H2SO4) and water between the electrodes. The sulfuric acid provides both the current conducting medium between the positive and negative electrodes, as well as an active material in the electrochemical reactions at the electrodes.
During discharge of the battery, the negative electrode is formed by the lead (Pb) being oxidized to Pb2+: Pb→Pb2++2e−.
The complete reaction is:Pb+H2SO4→PbSO4↓+2H++2e−.
Concurrently, the positive electrode is formed by Pb4+ being reduced to Pb2+ in the following reaction:Pb4++2e−→Pb2+.
The complete reaction is:PbO2+H2SO4+2H++2e−→PbSO4↓+2H2O.
As the lead-acid battery is discharged, lead (Pb) and lead dioxide (PbO2) are converted into lead sulfate (PbSO4). The water (2H2O) produced and the sulfuric acid (H2SO4) consumed dilute the electrolytic solution causing the lower density readings observed on a discharged battery.
Lead-acid batteries can be recharged using chargers falling into two broad classes: simple chargers, and closed loop or feedback chargers. Simple chargers deliver a low level charge current to the battery over a timed interval. The current level is chosen to prevent damage to the battery due to overcharging. Feedback chargers, on the other hand, monitor the state of the battery in order to control the magnitude of the charge current during the charge cycle. The charge cycle is composed of a high current phase and a regulation phase. During the high current phase, the feedback charger applies a high charge current to the battery in order to rapidly charge the battery. The feedback charger continues to monitor the state of the battery and reduces the charging current as the charge state of the battery is restored.
At room temperature the density of sulfuric acid is 1834 kg/m3, which is more than 1.8 times the density of water (1000 kg/m3). This difference in density can cause problems during recharging, particularly when the recharging occurs at a rapid rate, as the relatively higher density of the sulfuric acid causes it to settle downward relative to the water. This problem can be seen in more detail by examining the chemical reactions that occur during recharging. The downward arrows shown beside some products of the reaction indicate that these products are being deposited onto the plates
When the lead-acid battery is being charged, sulfuric acid is produced at both electrodes according to the following reactions:    At the positive electrode: PbSO4+2H2O→PbO2↓+H2SO4+2H++2e−    At the negative electrode: PbSO4+2H++2e−→Pb↓+H2SO4
As the sulfuric acid concentration rises near the positive and negative electrodes, the acid's higher density causes it to pour down the electrodes to raise the acid concentration at the bottom of the cell. This problem is called stratification.
Battery voltage depends on the acid concentration. Consequently, higher voltage is required near the bottom of a stratified cell to overcome the elevated equilibrium voltage and drive the charge reaction, leaving the bottom portion less charged. If the problem is not corrected, the conditions at the bottom of the electrodes will progressively deteriorate with every charge cycle performed. Eventually, the capacity of the cell will be irreversibly reduced.
In the prior art, this problem has been addressed by providing a 10 to 20% low rate overcharge at the end of every charge cycle. When the rate of electron flow (current) exceeds the rate of the main charge reaction, the unused electrons begin participating in irreversible side reactions (water electroysis):    The reaction at the positive electrode is 2H2O→O2+4H++4e−    The reaction at the negative electrode is 4H++4e−→2H2 
The relationship between current and amount of water decomposed can be evaluated using the Faraday constant (equivalent to 96485 As or 26.8 Ah). As the number of exchanged electrons in the electrolysis reaction is two, 53.6 Ah is required to decompose 1 molar weight of water (18 g). Thus, 1 Ah of overcharge current decomposes 0.336 g of water generating 0.68 liters of gas under normal conditions (at 25 degrees centigrade and normal atmospheric pressure). The gases produced by water electrolysis (H2 and 0), rise through the system causing an upward mixing movement of the liquid that ameliorates stratification. The intensity of the mixing movement of the gas is controlled by the rate of gassing or overcharge.
The solution of providing a 10 to 20% low rate overcharge at the end of every charge cycle suffers from the drawback that it is time-consuming, thereby largely canceling the advantages of rapid recharging. Also, a high overcharge rate tends to corrode the electrodes, thereby shortening the useful life of the battery. These effects are exacerbated at higher temperatures.
Thus, a method and apparatus for rapidly recharging batteries while avoiding stratification is desirable.